High Power Broadband Illumination Source

ABSTRACT

A system for generating broadband radiation is disclosed. The system includes a target material source configured to deliver one or more of a liquid or solid state target material to a plasma-forming region of a chamber. The system further includes a pump source configured to generate pump radiation to excite the target material in the plasma forming region of the chamber to generate broadband radiation. The system is further configured to transmit at least a portion of the broadband radiation generated in the plasma-forming region of the chamber out of the chamber through a windowless aperture.

PRIORITY

The present application claims priority to U.S. Provisional ApplicationSer. No. 62/536,914, filed Jul. 25, 2017, entitled HIGH POWER HIGHBRIGHTNESS VUV AND DUV BROADBAND CW AND MODULATED SOURCE AND METHOD OFPRODUCING THEREOF, naming Oleg Khodykin and Ilya Bezel as inventors,which is incorporated herein by reference in the entirety.

TECHNICAL FIELD

The present invention generally relates to plasma-based radiationsources, and, more particularly, to laser sustained plasma (LSP)broadband radiation sources including a plasma generated nearatmospheric pressure.

BACKGROUND

As the demand for integrated circuits having ever-smaller devicefeatures continues to increase, the need for improved illuminationsources used for inspection of these ever-shrinking devices continues togrow. One such illumination source includes a laser-sustained plasma(LSP) radiation source. Laser-sustained plasma light sources are capableof producing high-power broadband light. Laser-sustained plasma lightsources operate by focusing laser radiation into a gas volume in orderto excite the gas, such as argon or xenon, into a plasma state, which iscapable of emitting light. This effect is typically referred to as“pumping” the plasma. In current applications, a relatively high densityof plasma is needed and is achieved by providing high pressure (30-200atm) to a target gas.

The transmissive elements in an LSP radiation source experience highpressure and temperature due to the target gas within the chamberhousing the plasma. Transmissive windows, often made of materials suchas calcium fluoride, magnesium fluoride, or lithium fluoride, alsotransmit radiation below 190 nm that is generated by the plasma. Thecombination of high pressure, high temperature and radiation below 190nm make the lifetime of transmissive optics used in LSP radiationsources very short.

Therefore, it would be desirable to provide a system and method thatcure one or more shortfalls of the previous approaches identified above.

SUMMARY

An apparatus is disclosed, in accordance with one or more embodiments ofthe present disclosure. In one embodiment, the apparatus includes achamber. In another embodiment, the chamber is configured to contain avolume of buffer gas. In another embodiment, the apparatus includes atarget material source positioned on a first side of the chamber. Inanother embodiment, the apparatus includes a debris collector. Inanother embodiment, the apparatus includes a debris collector positionedon a second side of the chamber opposite the target material source. Inanother embodiment, the target material source is configured to delivera stream of target material through a plasma-forming region of thechamber. In another embodiment, the debris collector is configured tocollect target material. In another embodiment, the apparatus includes apump source. In another embodiment, the apparatus includes a pump sourceconfigured to deliver pump radiation to the plasma-forming region of thechamber. In another embodiment, the pump radiation from the pump sourceis sufficient to generate broadband radiation via formation of a plasmaby excitation of the target material within the plasma-forming region ofthe chamber. In another embodiment, the apparatus includes one or morefocusing optical elements. In another embodiment, the one or morefocusing optical elements are configured to focus the pump radiationinto the plasma-forming region. In another embodiment, the apparatusincludes one or more reflective collection optical elements. In anotherembodiment, the one or more reflective collection optical elements areconfigured to collect a portion of the broadband radiation from theplasma and deliver the portion of the broadband radiation to one or moreoptical elements external to the chamber through an aperture in a wallof the chamber.

A system is disclosed, in accordance with one or more embodiments of thepresent disclosure. In one embodiment, the system includes a broadbandsource. In another embodiment, the broadband source includes a chamberconfigured to contain a volume of inert gas. In another embodiment, thebroadband source includes a target material source positioned on a firstside of the chamber. In another embodiment, the broadband sourceincludes a debris collector positioned on a second side of the chamberopposite the target material source. In another embodiment, the targetmaterial source is configured to deliver a stream of target materialthrough a plasma-forming region of the chamber. In another embodiment,the debris collector is configured to collect target material. Inanother embodiment, the broadband source includes a pump source. Inanother embodiment, the pump source is configured to deliver pumpradiation to the plasma-forming region of the chamber. In anotherembodiment, the pump radiation from the pump source is sufficient togenerate broadband radiation via formation of a plasma by excitation ofthe target material within the plasma-forming region of the chamber. Inanother embodiment, the broadband source includes one or more focusingoptical elements. In another embodiment, the one or more focusingoptical elements are configured to focus the pump radiation into theplasma-forming region. In another embodiment, the broadband sourceincludes one or more reflective collection optical elements. In anotherembodiment, the one or more reflective collection optical elements areconfigured to collect a portion of the broadband radiation from theplasma and deliver the portion of the broadband radiation to one or moreoptical elements external to the chamber through an aperture in a wallof the chamber. In another embodiment, the broadband source includes aset of illuminator optics. In another embodiment, the set of illuminatoroptics are configured to direct the broadband radiation from the one ormore reflective collection optics to one or more specimens. In anotherembodiment, the broadband source includes a detector. In anotherembodiment, the broadband source includes a set of projection optics. Inanother embodiment, the set of projection optics are configured toreceive illumination from the surface of the one or more specimens anddirect the illumination from the one or more specimens to the detector.

A method is disclosed, in accordance with one or more embodiments of thepresent disclosure. In one embodiment, the method includes delivering astream of target material through a plasma-forming region of a gaschamber. In another embodiment, the method includes collecting debrisfrom the plasma-forming region. In another embodiment, the methodincludes generating pump radiation. In another embodiment, the methodincludes focusing the pump radiation into the plasma-forming region ofthe chamber to generate broadband radiation via formation of a plasma byexcitation of the target material within the plasma-forming region ofthe chamber. In another embodiment, the method includes collecting aportion of the broadband radiation from the plasma and delivering theportion of the broadband radiation to one or more optical elementsexternal to the chamber through a windowless aperture in a wall of thechamber.

BRIEF DESCRIPTION OF THE DRAWINGS

The numerous advantages of the disclosure may be better understood bythose skilled in the art by reference to the accompanying figures inwhich:

FIG. 1 illustrates a simplified schematic view of a laser sustainedplasma (LSP) broadband radiation source, in accordance with one or moreembodiments of the present disclosure;

FIG. 2A illustrates a pump and collection configuration including a lowNA pump and high NA collection, in accordance with one or moreembodiments of the present disclosure;

FIG. 2B illustrates a pump and collection configuration including a highNA pump and low NA collection, in accordance with one or moreembodiments of the present disclosure;

FIG. 3 illustrates a simplified schematic view of an opticalcharacterization system implementing the LSP radiation source, inaccordance with one or more embodiments of the present disclosure; and

FIG. 4 illustrates a flow diagram depicting a method for generatingbroadband radiation, in accordance with one or more embodiments of thepresent disclosure.

DETAILED DESCRIPTION

The present disclosure has been particularly shown and described withrespect to certain embodiments and specific features thereof. Theembodiments set forth herein are taken to be illustrative rather thanlimiting. It should be readily apparent to those of ordinary skill inthe art that various changes and modifications in form and detail may bemade without departing from the spirit and scope of the disclosure.

The present disclosure generally relates to a laser-sustained plasma(LSP) radiation source. The LSP includes a material source that providesone or more target materials (e.g., argon, xenon, neon, or helium) in anon-gaseous form for laser excitation. For example, the LSP may providetarget material in the form of at least a liquid, a solid or acombination of a liquid and a solid. For instance, target materialprovided by the material source may be provided by one or more of a jet,a stream, a slurry, a mist, a spray, a droplet, a bead, a grain, aparticle, or another other non-gaseous form of material. The plasmagenerated by the LSP emits broadband radiation that is directed byreflective collection optics out of the LSP through a windowlessaperture. It is noted herein that an advantage of the present disclosureis the utilization of reflective optics that eliminate the need fortransmissive windows. It is noted herein that a further advantage of thepresent disclosure is that delivery of a high density target material inthe form of a solid, a liquid, or a combination of a solid and liquideliminates the need for a high pressure gas chamber.

Reference will now be made in detail to the subject matter disclosed,which is illustrated in the accompanying drawings.

Referring generally to FIGS. 1-5, systems and methods for generatingimproved laser-sustained plasma (LSP) radiation sources are described,in accordance with one or more embodiments of the present disclosure.Embodiments of the present disclosure are directed to the delivery oftarget material in the form of a liquid jet, liquid droplets, a frozenjet, frozen droplets, a slurry, a stream or a combination of thesetarget material forms. Additional embodiments of the present disclosureare directed to a liquid jet of 10-2000 microns in diameter and/ordroplets of about 10-300 m/s speed. Additional embodiments of thepresent disclosure are directed to delivery of the target material at ornear atmospheric pressure (e.g., 0.1-2 atm). Additional embodiments ofthe present disclosure utilize reflective collection optics to transmitbroadband radiation from the LSP radiation source through a windowlessaperture. It is noted herein that enhanced, fast-flow of the targetmaterial within the chamber may promote stable plasma 116 generation.Additional embodiments of the present disclosure collect broadbandradiation, such as, but not limited to, VUV and/or DUV radiation withhigh numerical aperture (NA) (e.g., about π srad) broadband reflectiveoptics. It is further noted herein, that in order for an LSP radiationsource to have significant brightness in DUV and/or VUV, a relativelyhigh density of the plasma is needed. For example, density of an LSPradiation source should be sufficiently high so as to absorb the pumplaser beam and also sufficiently dense to provide sufficient emissivityin DUV and/or VUV and other spectral regions. Embodiments of the presentdisclosure deliver the working gas at high density to the plasma in formof liquid or solid jet.

FIG. 1 illustrates a simplified schematic view of a broadband LSPradiation source 100, in accordance with one or more embodiments of thepresent disclosure. In one embodiment, the LSP radiation source 100includes a target material source 102, a pump source 108, pump laserfocusing optics 110, and a set of collection optics 120. In anotherembodiment, the LSP radiation source 100 includes a debris collector124. It is noted that the LSP radiation source 100 may generatebroadband radiation 118 of any wavelength range including, but notlimited to, vacuum ultraviolet (VUV) (e.g., 100-190 nm) and/or deepultraviolet (DUV) (e.g., 190-260 nm).

In one embodiment, the target material source 102 delivers one or moretarget materials 104 into the chamber 106. For example, the targetmaterial source 102 may introduce one or more target materials 104 intothe chamber 106 in the form of a liquid jet, liquid droplets, a frozenjet, frozen droplets, or a combination of these target material forms.In another embodiment, the stream delivery parameters of target material104 from the target material source 102 are adjusted such that eitherall material delivered by the target material source 102 evaporates inthe plasma region or some of the material passes through the plasma andis collected by the debris collector 124. In another embodiment, thedebris collector 124 is positioned on a side of the chamber opposite thetarget material source 102. It is noted herein that adjustment of streamdelivery parameters may promote stable plasma 116 generation andgenerate broadband radiation 118 with one or more substantially constantproperties.

The target material source 102 may deliver any type of target materialknown in the art of LSP broadband sources. For example, the targetmaterial may include, but is not limited to, Ar, Xe, Ne, and He ormixtures of Ar, Xe, Ne, and He.

In one embodiment, the pump source 108 is configured to generate a pumpbeam 112 (e.g., laser radiation) focused by pump laser focusing optics110. In another embodiment, the pump source 108 includes any radiationsource known in the art including, but not limited to, one or morelasers. In another embodiment, the pump beam 112 includes radiation ofany wavelength or wavelength range known in the art including, but notlimited to, infrared (IR) radiation, near infrared (NIR) radiation,ultraviolet (UV) radiation, visible radiation, or other radiationsuitable to form a plasma when incident on a suitable target material.

In one embodiment, the pump laser focusing optics 110 focus the pumpbeam 112 through a pump laser window 114 into the chamber 106. Inanother embodiment, the pump laser focusing optics 110 focus the pumpbeam 112 into one or more target materials 104 so as to generate and/orsustain a plasma 116. In another embodiment, target material 104 isevaporated and ionized by the pump beam 112 providing a high localconcentration of plasma in the chamber. In another embodiment, thepressure of heated material ejected from the plasma will rapidlydecrease from the plasma region outward. It is noted herein that thepump laser focusing optics 110 may include any optical element known inthe art for directing and/or focusing radiation including, but notlimited to, a lens, a mirror, a prism, a polarizer, a grating, a filter,or a beamsplitter.

The focusing of the pump beam 112 into the target material 104 causesenergy to be absorbed through one or more absorption lines of the targetmaterial and/or plasma 116 contained within chamber 106, thereby“pumping” the one or more target material 104 in order to generateand/or sustain the plasma 116. For example, the pump laser focusingoptics 110 may generate and/or sustain a plasma 116 by focusing the pumpbeam 112 to one or more focal points within the one or more targetmaterials 104 contained within the chamber 106 in order to generateand/or sustain a plasma 116. It is noted herein that the LSP radiationsource 100 may include one or more additional ignition sources used tofacilitate the generation of the plasma 116 without departing from thespirit or scope of the present disclosure. For example, the chamber 106may include one or more electrodes which may initiate and/or maintainthe plasma 116.

In one embodiment, the broadband radiation 118 generated by the plasma116 exits the chamber 106 through one or more apertures 122. Forexample, the collection optics 120 may be arranged so as to collectbroadband radiation 118 from the plasma 116 and, in turn, direct atleast a portion of the collected broadband plasma through the one ormore apertures 122. In another embodiment, the one or more apertures 122are windowless and located in the wall of the chamber 106. For example,the one or more apertures 122 may include, but are not limited to, ahole, a port, an outlet, a vent, a space or any other opening allowingthe broadband radiation 118 to exit the chamber 106 through the wall ofthe chamber 106 without being transmitted through a material other thanambient atmosphere.

In one embodiment, the chamber 106 is fluidically coupled to a vacuumpump 126. In another embodiment, the pressure in the chamber 106 ismaintained about 1 atm. For example, the pressure in the chamber 106 maybe kept in a range of 0.1-2 atm. For instance, the vacuum pump 126 mayremove gas (e.g., gas ejected from the plasma, buffer gas) from thechamber 106 to maintain a pressure in a range of 0.1-2 atm in thechamber 106.

In one embodiment, the set of collection optics 120 include one or moreoptical elements known in the art configured to collect radiation (e.g.,broadband radiation 118) including, but not limited to, one or moremirrors, one or more prisms, one or more lenses, one or more diffractiveoptical elements, one or more parabolic mirrors, one or more ellipticalmirrors, and the like. It is noted herein that the set of collectionoptics 120 may be configured to collect and/or focus broadband radiation118 generated by plasma 116 to be used for one or more down-streamprocesses including, but not limited to, imaging processes, inspectionprocesses, metrology processes, lithography processes, and the like. Inanother embodiment, the set of collection optics 120 are protected fromdamage by being positioned at a sufficient distance from the plasma 116.For example, the set of collection optics 120 may be positioned in arange of 5-100 cm from the plasma so as not to be damaged by the plasma116.

In one embodiment, the pump laser focusing optics 110 and the collectionoptics 120 are physically separated. It is noted that physicalseparation of the pump laser focusing optics 110 and the collectionoptics 120 eliminate the need for cold mirror and/or dual bandwidthreflective elliptical optics.

In one embodiment, the reflective optic surfaces of the collectionoptics 120 in the chamber 106 are protected by a buffer gas. Forexample, a buffer gas (e.g., inert gas, same material as the targetmaterial, different material from the target material) is maintained inthe chamber 106 at about 1 atm.

In one embodiment, the specific geometry of the pump source focusingoptics 110 and the collection optics 120 is optimized depending on thelaser power and collection etendue requirements. In another embodiment,collection optics 120 direct broadband radiation to a collectionlocation 128.

In one embodiment, the pump source 108 includes one or more radiationsources. For example, the pump source 108 may include any laser systemknown in the art. For instance, the pump source 108 may include anylaser system known in the art capable of emitting radiation in theinfrared, visible and/or ultraviolet portions of the electromagneticspectrum. In another embodiment, the pump source 108 includes a lasersystem configured to emit continuous wave (CW) laser radiation. Forexample, the pump source 108 may include one or more CW infrared lasersources.

In another embodiment, the pump source 108 includes one or moremodulated CW lasers configured to provide modulated laser light to theplasma 116. In another embodiment, the pump source 108 may include oneor more pulsed lasers configured to provide pulsed laser light to theplasma 116.

In one embodiment, the pump source 108 may include one or more diodelasers. For example, the pump source 108 may include one or more diodelasers emitting radiation at a wavelength corresponding with any one ormore absorption lines of the species of the target material 104contained within the chamber 106. It is noted that a diode laser of thepump source 108 may be selected for implementation such that thewavelength of the diode laser is tuned to any absorption line of anyplasma (e.g., ionic transition line) or any absorption line of theplasma-producing target material (e.g., highly excited neutraltransition line) known in the art. As such, the choice of a given diodelaser (or set of diode lasers) will depend on the type of targetmaterial contained within the chamber 106 of system 100.

In another embodiment, the pump source 108 includes an ion laser. Forexample, the pump source 108 may include any noble gas ion laser knownin the art. For instance, in the case of an argon-based plasma, the pumpsource 108 used to pump argon ions may include an Ar+ laser.

In another embodiment, the pump source 108 includes one or morefrequency converted laser systems. For example, the pump source 108 mayinclude an Nd:YAG or Nd:YLF laser.

In another embodiment, the pump source 108 includes one or morenon-laser sources. In a general sense, the pump source 108 may includeany non-laser light source known in the art. For instance, the pumpsource 108 may include any non-laser system known in the art capable ofemitting radiation discretely or continuously in the infrared, visibleor ultraviolet portions of the electromagnetic spectrum.

In another embodiment, the pump source 108 includes two or moreradiation sources. For example, the pump source 108 may include, but isnot limited to, two or more lasers. For instance, the pump source 108(or pump sources) may include multiple diode lasers. In anotherinstance, the pump source 108 may include multiple CW lasers and/orpulsed lasers. In another embodiment, each of two or more lasers mayemit laser radiation tuned to a different absorption line of the targetmaterial or plasma within the chamber 106 of system 100.

In one embodiment, the pump source 108 generates pump radiation with apulse spacing of 100-1000 ns. In another embodiment, the pump source 108operates at a power in the range of 3-100 kW. In another embodiment, thepump source 108 includes a peak laser intensity of greater than 10,000W/cm². For example, the pump source 108 may include a peak laserintensity of greater than 10⁵ W/cm². In another embodiment, ignition ofthe one or more target material 104 to generate a plasma 116 isperformed using a short pulse (e.g., <100 ns) high peak power (>10⁸W/cm²) laser.

FIG. 2A illustrates a pump and collection configuration including a lowNA pump and high NA collection, in accordance with one or moreembodiments of the present disclosure. In one embodiment, the pumpsource 108, the one or more focusing optical elements 110, and thereflective collection optical elements 120 are arranged such that thebroadband radiation 118 has a numerical aperture higher than the NA ofthe pump radiation 112. In another embodiment, collection optics 120direct broadband radiation to a collection location 128.

FIG. 2B illustrates a pump and collection configuration including a highNA pump and low NA collection, in accordance with one or moreembodiments of the present disclosure. In one embodiment, the pumpsource 108, the one or more focusing optical elements 110, and thereflective collection optical elements 120 are arranged such that thebroadband radiation 118 has a numerical aperture lower than the NA ofthe pump radiation. In another embodiment, collection optics 120 directbroadband radiation to a collection location 128.

FIG. 3 illustrates a simplified schematic view of an opticalcharacterization system 300 implementing the LSP radiation source 100,in accordance with one or more embodiments of the present disclosure. Inone embodiment, system 300 includes the LSP radiation source 100, anillumination arm 303, a collection arm 305, a detector 314, and acontroller 318 including one or more processors 320 and memory 322.

It is noted herein that system 300 may comprise any imaging, inspection,metrology, lithography, or other characterization system known in theart. In this regard, system 300 may be configured to perform inspection,optical metrology, lithography, and/or any form of imaging on a specimen307. Specimen 307 may include any sample known in the art including, butnot limited to, a wafer, a reticle, a photomask, and the like. It isnoted that system 300 may incorporate one or more of the variousembodiments of the LSP radiation source 100 described throughout thepresent disclosure.

In one embodiment, specimen 307 is disposed on a stage assembly 312 tofacilitate movement of specimen 307. Stage assembly 312 may include anystage assembly 312 known in the art including, but not limited to, anX-Y stage, an R-θ stage, and the like. In another embodiment, stageassembly 312 is capable of adjusting the height of specimen 307 duringinspection or imaging to maintain focus on the specimen 307.

In one embodiment, the illumination arm 303 is configured to directbroadband radiation 118 from the LSP radiation source 100 to thespecimen 307. The illumination arm 303 may include any number and typeof optical components known in the art. In one embodiment, theillumination arm 303 includes one or more optical elements 302, a beamsplitter 304, and an objective lens 306. In this regard, illuminationarm 303 may be configured to focus broadband radiation 118 from the LSPradiation source 100 onto the surface of the specimen 307. The one ormore optical elements 302 may include any optical element or combinationof optical elements known in the art including, but not limited to, oneor more mirrors, one or more lenses, one or more polarizers, one or moregratings, one or more filters, one or more beam splitters, and the like.It is noted herein that the collection location 128 may include, but isnot limited to, one or more of the optical elements 302, a beam splitter304, or an objective lens 306.

In one embodiment, system 300 includes a collection arm 305 configuredto collect light reflected, scattered, diffracted, and/or emitted fromspecimen 307. In another embodiment, collection arm 305 may directand/or focus the light from the specimen 307 to a sensor 316 of adetector assembly 314. It is noted that sensor 316 and detector assembly314 may include any sensor and detector assembly known in the art. Thesensor 316 may include, but is not limited to, a CCD sensor or a CCD-TDIsensor. Further, sensor 316 may include, but is not limited to, a linesensor or an electron-bombarded line sensor.

In one embodiment, detector assembly 314 is communicatively coupled to acontroller 318 including one or more processors 320 and memory 322. Forexample, the one or more processors 320 may be communicatively coupledto memory 322, wherein the one or more processors 320 are configured toexecute a set of program instructions stored on memory 322. In oneembodiment, the one or more processors 320 are configured to analyze theoutput of detector assembly 314. In one embodiment, the set of programinstructions are configured to cause the one or more processors 320 toanalyze one or more characteristics of specimen 307. In anotherembodiment, the set of program instructions are configured to cause theone or more processors 320 to modify one or more characteristics ofsystem 300 in order to maintain focus on the specimen 307 and/or thesensor 316. For example, the one or more processors 320 may beconfigured to adjust the objective lens 306 or one or more opticalelements 302 in order to focus broadband radiation 118 from LSPradiation source 100 onto the surface of the specimen 307. By way ofanother example, the one or more processors 320 may be configured toadjust the objective lens 306 and/or one or more optical elements 310 inorder to collect illumination from the surface of the specimen 307 andfocus the collected illumination on the sensor 316.

It is noted that the system 300 may be configured in any opticalconfiguration known in the art including, but not limited to, adark-field configuration, a bright-field orientation, and the like.

It is noted herein that the one or more components of system 100 may becommunicatively coupled to the various other components of system 100 inany manner known in the art. For example, the LSP radiation source 100,detector assembly 314, controller 318, and one or more processors 320may be communicatively coupled to each other and other components via awireline (e.g., copper wire, fiber optic cable, and the like) orwireless connection (e.g., RF coupling, IR coupling, data networkcommunication (e.g., WiFi, WiMax, Bluetooth and the like).

Additional details of various embodiments of optical characterizationsystem 300 are described in U.S. patent application Ser. No. 13/554,954,entitled “Wafer Inspection System,” filed on Jul. 9, 2012; U.S.Published Patent Application 2009/0180176, entitled “Split FieldInspection System Using Small Catadioptric Objectives,” published onJul. 16, 2009; U.S. Published Patent Application 2007/0002465, entitled“Beam Delivery System for Laser Dark-Field Illumination in aCatadioptric Optical System,” published on Jan. 4, 2007; U.S. Pat. No.5,999,310, entitled “Ultra-broadband UV Microscope Imaging System withWide Range Zoom Capability,” issued on Dec. 7, 1999; U.S. Pat. No.7,525,649 entitled “Surface Inspection System Using Laser LineIllumination with Two Dimensional Imaging,” issued on Apr. 28, 2009;U.S. Published Patent Application 2013/0114085, entitled “DynamicallyAdjustable Semiconductor Metrology System,” by Wang et al. and publishedon May 9, 2013; U.S. Pat. No. 5,608,526, entitled “Focused BeamSpectroscopic Ellipsometry Method and System, by Piwonka-Corle et al.,issued on Mar. 4, 1997; and U.S. Pat. No. 6,297,880, entitled “Apparatusfor Analyzing Multi-Layer Thin Film Stacks on Semiconductors,” byRosencwaig et al., issued on Oct. 2, 2001, which are each incorporatedherein by reference in their entirety.

The one or more processors 320 of the present disclosure may include anyone or more processing elements known in the art. In this sense, the oneor more processors 320 may include any microprocessor-type deviceconfigured to execute software algorithms and/or instructions. In oneembodiment, the one or more processors 320 may consist of a desktopcomputer, mainframe computer system, workstation, image computer,parallel processor, or other computer system (e.g., networked computer)configured to execute a program configured to operate the system 300and/or LSP radiation source 100, as described throughout the presentdisclosure. It should be recognized that the steps described throughoutthe present disclosure may be carried out by a single computer systemor, alternatively, multiple computer systems. In general, the term“processor” may be broadly defined to encompass any device having one ormore processing elements, which execute program instructions from anon-transitory memory medium 322. Moreover, different subsystems of thevarious systems disclosed may include processor or logic elementssuitable for carrying out at least a portion of the steps describedthroughout the present disclosure. Therefore, the above descriptionshould not be interpreted as a limitation on the present disclosure butmerely an illustration.

The memory medium 322 may include any storage medium known in the artsuitable for storing program instructions executable by the associatedone or more processors 320. For example, the memory medium 322 mayinclude a non-transitory memory medium. For instance, the memory medium322 may include, but is not limited to, a read-only memory, a randomaccess memory, a magnetic or optical memory device (e.g., disk), amagnetic tape, a solid state drive, and the like. In another embodiment,the memory 322 is configured to store one or more results and/or outputsof the various steps described herein. It is further noted that memory322 may be housed in a common controller housing with the one or moreprocessors 320. In an alternative embodiment, the memory 322 may belocated remotely with respect to the physical location of the processors320. For instance, the one or more processors 320 may access a remotememory (e.g., server), accessible through a network (e.g., internet,intranet, and the like). In another embodiment, the memory medium 322maintains program instructions for causing the one or more processors320 to carry out the various steps described through the presentdisclosure.

In one embodiment, the system 300 may include a user interface (notshown). In one embodiment, the user interface is communicatively coupledto the one or more processors 320. In another embodiment, the userinterface device may be utilized to accept selections and/orinstructions from a user. In some embodiments, described further herein,a display may be used to display data to a user. In turn, a user mayinput selection and/or instructions (e.g., selection, sizing, and/orposition of filter box) responsive to data displayed to the user via thedisplay device.

The user interface device may include any user interface known in theart. For example, the user interface may include, but is not limited to,a keyboard, a keypad, a touchscreen, a lever, a knob, a scroll wheel, atrack ball, a switch, a dial, a sliding bar, a scroll bar, a slide, ahandle, a touch pad, a paddle, a steering wheel, a joystick, a bezelmounted input device, or the like. In the case of a touchscreeninterface device, those skilled in the art should recognize that a largenumber of touchscreen interface devices may be suitable forimplementation in the present invention. For instance, the displaydevice may be integrated with a touchscreen interface, such as, but notlimited to, a capacitive touchscreen, a resistive touchscreen, a surfaceacoustic based touchscreen, an infrared based touchscreen, or the like.In a general sense, any touchscreen interface capable of integrationwith the display portion of a display device is suitable forimplementation in the present disclosure.

The display device may include any display device known in the art. Inone embodiment, the display device may include, but is not limited to, aliquid crystal display (LCD), an organic light-emitting diode (OLED)based display or a CRT display. Those skilled in the art shouldrecognize that a variety of display devices may be suitable forimplementation in the present disclosure and the particular choice ofdisplay device may depend on a variety of factors, including, but notlimited to, form factor, cost, and the like. In a general sense, anydisplay device capable of integration with a user interface device(e.g., touchscreen, bezel mounted interface, keyboard, mouse, trackpad,and the like) is suitable for implementation in the present disclosure.

In some embodiments, the LSP radiation source 100 and system 300, asdescribed herein, may be configured as a “stand alone tool” or a toolthat is not physically coupled to a process tool. In other embodiments,such an inspection or metrology system may be coupled to a process tool(not shown) by a transmission medium, which may include wired and/orwireless portions. The process tool may include any process tool knownin the art such as a lithography tool, an etch tool, a deposition tool,a polishing tool, a plating tool, a cleaning tool, or an ionimplantation tool. The results of inspection or measurement performed bythe systems described herein may be used to alter a parameter of aprocess or a process tool using a feedback control technique, afeedforward control technique, and/or an in-situ control technique. Theparameter of the process or the process tool may be altered manually orautomatically.

The embodiments of the LSP radiation source 100 and system 300 may befurther configured as described herein. In addition, the LSP radiationsource 100 and system 300 may be configured to perform any other step(s)of any of the method embodiment(s) described herein.

FIG. 4 illustrates a flow diagram depicting a method 400 for generatingbroadband radiation 118, in accordance with one or more embodiments ofthe present disclosure. It is noted herein that the steps of method 400may be implemented all or in part by LSP radiation source 100. It isfurther recognized, however, that the method 400 is not limited to theLSP radiation source 100 in that additional or alternative system-levelembodiments may carry out all or part of the steps of method 400.

In step 402, a stream of target material 104 is delivered through aplasma-forming region of a chamber 106. In one embodiment, the targetmaterial source 102 introduces one or more target materials 104 into thechamber 106 in the form of a liquid jet, liquid droplets, a frozen jet,frozen droplets, or a combination of these target material forms. Forexample, the target material source 102 may deliver one or more ofargon, xenon, neon, or helium in one or more of a solid or liquid stateinto the chamber 106 to generate and/or sustain a plasma. For instance,the target material source 102 may be configured to deliver the streamof target material at a speed between 10 and 300 m/s through theplasma-forming region. By way of another instance, the target materialsource 102 may be configured to deliver a stream of target material thathas a diameter between 10 and 2000 μm through the plasma-forming region.

In step 404, debris is collected by a debris collector. For example,plasma-forming target material 104 from step 402 not consumed by theplasma 116 is collected by a debris collector 124. In one embodiment,the stream delivery parameters are adjusted such that either allmaterial delivered by the target material source 102 evaporates in theplasma region or some of the material passes through the plasma and iscollected by the debris collector 124. It is noted herein thatadjustment of stream delivery parameters may promote stable plasma 116generation and generate broadband radiation 118 with one or moresubstantially constant properties.

In step 406, the pump source 108 generates pump radiation 112. In oneembodiment, the pump source 108 generates pump radiation 112 focused bypump laser focusing optics 110 into the chamber 106. For example, thepump source 108 may include one or more lasers to generate pumpradiation 112 directed into the chamber 106. By way of another example,one or more non-laser sources may generate pump radiation 112 directedinto the chamber 106.

In step 408, pump radiation 112 is focused into the plasma-formingregion of the chamber 106 to generate broadband radiation 118 viaformation of a plasma 116 by excitation of the target material 104within the plasma-forming region of the chamber 106. For example, a pumpsource 108 may direct pump radiation 112 into a chamber 106 to generatea plasma 116. In another embodiment, the LSP radiation source 100generates radiation including, but not limited to, broadband radiation118. For example, the LSP radiation source 100 may generate broadbandradiation 118 in the range of vacuum ultraviolet (VUV) (100-190 nm) anddeep ultraviolet (DUV) (190-260 nm). It is noted herein that the chamber106 may include one or more additional ignition sources (e.g.,electrodes) configured to initiate and/or maintain the plasma 116.

In step 410, a portion of the broadband radiation 118 from the plasma116 is collected and delivered to one or more optical elements externalto the chamber 106 at a collection location 128 through a windowlessaperture 122 in a wall of the chamber 106. For example, broadbandradiation 118 generated by plasma 116 in the chamber 106 may becollected by collection optics 120 and directed through aperture 122 tocollection location 128 where external optical elements receive thebroadband radiation 118.

One skilled in the art will recognize that the herein describedcomponents (e.g., operations), devices, objects, and the discussionaccompanying them are used as examples for the sake of conceptualclarity and that various configuration modifications are contemplated.Consequently, as used herein, the specific exemplars set forth and theaccompanying discussion are intended to be representative of their moregeneral classes. In general, use of any specific exemplar is intended tobe representative of its class, and the non-inclusion of specificcomponents (e.g., operations), devices, and objects should not be takenas limiting.

Those having skill in the art will appreciate that there are variousvehicles by which processes and/or systems and/or other technologiesdescribed herein can be effected (e.g., hardware, software, and/orfirmware), and that the preferred vehicle will vary with the context inwhich the processes and/or systems and/or other technologies aredeployed. For example, if an implementer determines that speed andaccuracy are paramount, the implementer may opt for a mainly hardwareand/or firmware vehicle; alternatively, if flexibility is paramount, theimplementer may opt for a mainly software implementation; or, yet againalternatively, the implementer may opt for some combination of hardware,software, and/or firmware. Hence, there are several possible vehicles bywhich the processes and/or devices and/or other technologies describedherein may be effected, none of which is inherently superior to theother in that any vehicle to be utilized is a choice dependent upon thecontext in which the vehicle will be deployed and the specific concerns(e.g., speed, flexibility, or predictability) of the implementer, any ofwhich may vary.

The previous description is presented to enable one of ordinary skill inthe art to make and use the invention as provided in the context of aparticular application and its requirements. As used herein, directionalterms such as “top,” “bottom,” “over,” “under,” “upper,” “upward,”“lower,” “down,” and “downward” are intended to provide relativepositions for purposes of description, and are not intended to designatean absolute frame of reference. Various modifications to the describedembodiments will be apparent to those with skill in the art, and thegeneral principles defined herein may be applied to other embodiments.Therefore, the present invention is not intended to be limited to theparticular embodiments shown and described, but is to be accorded thewidest scope consistent with the principles and novel features hereindisclosed.

With respect to the use of substantially any plural and/or singularterms herein, those having skill in the art can translate from theplural to the singular and/or from the singular to the plural as isappropriate to the context and/or application. The varioussingular/plural permutations are not expressly set forth herein for sakeof clarity.

All of the methods described herein may include storing results of oneor more steps of the method embodiments in memory. The results mayinclude any of the results described herein and may be stored in anymanner known in the art. The memory may include any memory describedherein or any other suitable storage medium known in the art. After theresults have been stored, the results can be accessed in the memory andused by any of the method or system embodiments described herein,formatted for display to a user, used by another software module,method, or system, and the like. Furthermore, the results may be stored“permanently,” “semi-permanently,” temporarily,” or for some period oftime. For example, the memory may be random access memory (RAM), and theresults may not necessarily persist indefinitely in the memory.

It is further contemplated that each of the embodiments of the methoddescribed above may include any other step(s) of any other method(s)described herein. In addition, each of the embodiments of the methoddescribed above may be performed by any of the systems described herein.

Embodiments of the present disclosure are directed to a buoyancy-drivenclosed recirculation gas loop for facilitating fast gas flow through inan LSP radiation source. Advantageously, the LSP radiation source 100 ofthe present disclosure may include fewer mechanically actuatedcomponents than do previous approaches. Thus, the LSP radiation source100 of the present disclosure may produce less noise, require smallervolumes of gas, and require lower maintenance costs and safetymanagement.

The herein described subject matter sometimes illustrates differentcomponents contained within, or connected with, other components. It isto be understood that such depicted architectures are merely exemplary,and that in fact many other architectures can be implemented whichachieve the same functionality. In a conceptual sense, any arrangementof components to achieve the same functionality is effectively“associated” such that the desired functionality is achieved. Hence, anytwo components herein combined to achieve a particular functionality canbe seen as “associated with” each other such that the desiredfunctionality is achieved, irrespective of architectures or intermedialcomponents. Likewise, any two components so associated can also beviewed as being “connected,” or “coupled,” to each other to achieve thedesired functionality, and any two components capable of being soassociated can also be viewed as being “couplable,” to each other toachieve the desired functionality. Specific examples of couplableinclude but are not limited to physically mateable and/or physicallyinteracting components and/or wirelessly interactable and/or wirelesslyinteracting components and/or logically interacting and/or logicallyinteractable components.

Furthermore, it is to be understood that the invention is defined by theappended claims. It will be understood by those within the art that, ingeneral, terms used herein, and especially in the appended claims (e.g.,bodies of the appended claims) are generally intended as “open” terms(e.g., the term “including” should be interpreted as “including but notlimited to,” the term “having” should be interpreted as “having atleast,” the term “includes” should be interpreted as “includes but isnot limited to,” and the like). It will be further understood by thosewithin the art that if a specific number of an introduced claimrecitation is intended, such an intent will be explicitly recited in theclaim, and in the absence of such recitation no such intent is present.For example, as an aid to understanding, the following appended claimsmay contain usage of the introductory phrases “at least one” and “one ormore” to introduce claim recitations. However, the use of such phrasesshould not be construed to imply that the introduction of a claimrecitation by the indefinite articles “a” or “an” limits any particularclaim containing such introduced claim recitation to inventionscontaining only one such recitation, even when the same claim includesthe introductory phrases “one or more” or “at least one” and indefinitearticles such as “a” or “an” (e.g., “a” and/or “an” should typically beinterpreted to mean “at least one” or “one or more”); the same holdstrue for the use of definite articles used to introduce claimrecitations. In addition, even if a specific number of an introducedclaim recitation is explicitly recited, those skilled in the art willrecognize that such recitation should typically be interpreted to meanat least the recited number (e.g., the bare recitation of “tworecitations,” without other modifiers, typically means at least tworecitations, or two or more recitations). Furthermore, in thoseinstances where a convention analogous to “at least one of A, B, and C,and the like” is used, in general such a construction is intended in thesense one having skill in the art would understand the convention (e.g.,“a system having at least one of A, B, and C” would include but not belimited to systems that have A alone, B alone, C alone, A and Btogether, A and C together, B and C together, and/or A, B, and Ctogether, and the like). In those instances where a convention analogousto “at least one of A, B, or C, and the like” is used, in general such aconstruction is intended in the sense one having skill in the art wouldunderstand the convention (e.g., “a system having at least one of A, B,or C” would include but not be limited to systems that have A alone, Balone, C alone, A and B together, A and C together, B and C together,and/or A, B, and C together, and the like). It will be furtherunderstood by those within the art that virtually any disjunctive wordand/or phrase presenting two or more alternative terms, whether in thedescription, claims, or drawings, should be understood to contemplatethe possibilities of including one of the terms, either of the terms, orboth terms. For example, the phrase “A or B” will be understood toinclude the possibilities of “A” or “B” or “A and B.”

It is believed that the present disclosure and many of its attendantadvantages will be understood by the foregoing description, and it willbe apparent that various changes may be made in the form, constructionand arrangement of the components without departing from the disclosedsubject matter or without sacrificing all of its material advantages.The form described is merely explanatory, and it is the intention of thefollowing claims to encompass and include such changes. Furthermore, itis to be understood that the invention is defined by the appendedclaims.

1. An apparatus comprising: a chamber configured to contain a volume ofbuffer gas; a target material source positioned on a first side of thechamber; a debris collector positioned on a second side of the chamberopposite the target material source, wherein the target material sourceis configured to deliver a stream of target material through aplasma-forming region of the chamber, wherein the debris collector isconfigured to collect target material; a pump source configured todeliver pump radiation to the plasma-forming region of the chamber,wherein the pump radiation is sufficient to generate broadband radiationvia formation of a plasma by excitation of the target material withinthe plasma-forming region of the chamber; one or more focusing opticalelements configured to focus the pump radiation into the plasma-formingregion; and one or more reflective collection optical elementsconfigured to collect a portion of the broadband radiation from theplasma and deliver the portion of the broadband radiation to one or moreoptical elements external to the chamber through an aperture in a wallof the chamber.
 2. The apparatus of claim 1, wherein the broadbandradiation comprises: at least one of vacuum ultraviolet (VUV) or deepultraviolet (DUV) radiation.
 3. The apparatus of claim 1, wherein thepump source, the one or more focusing optical elements and thereflective collection optical elements are arranged such that thebroadband radiation has a numerical aperture higher than the NA of thepump radiation.
 4. The apparatus of claim 1, wherein the pump source,the one or more focusing optical elements and the reflective collectionoptical elements are arranged such that the broadband radiation has anumerical aperture lower than the NA of the pump radiation.
 5. Theapparatus of claim 1, wherein the aperture in the wall of the chamber iswindowless.
 6. The apparatus of claim 1, wherein the stream of targetmaterial comprises: at least one of a stream of liquid target material,a stream of solid target material, or a series of droplets of the targetmaterial.
 7. The apparatus of claim 1, wherein the target materialsource is configured to deliver the stream of target material at a speedbetween 10 and 300 m/s.
 8. The apparatus of claim 1, wherein the streamof target material has a diameter between 10 and 2000 μm.
 9. Theapparatus of claim 1, wherein the target material comprises: at leastone of argon, xenon, neon, or helium.
 10. The apparatus of claim 9,wherein the target material comprises: a mixture containing at least oneof argon, xenon, neon, or helium.
 11. The apparatus of claim 1, whereinthe buffer gas comprises: an inert gas.
 12. The apparatus of claim 1,wherein the buffer gas is the same as the target material.
 13. Theapparatus of claim 1, wherein the buffer gas is different from thetarget material.
 14. The apparatus of claim 1, wherein the chamber isconfigured to contain the gas at a pressure between 0.1 and 2.0 atm. 15.The apparatus of claim 1, wherein the pump source comprises: at leastone of a continuous-wave (CW) laser, a pulsed laser, or a modulated CWlaser.
 16. The apparatus of claim 15, wherein the pump source isconfigured to generate pump radiation with pulse spacing of 100 to 1000nanoseconds.
 17. The apparatus of claim 15, wherein the pump source isconfigured to generate pump radiation at a power between 3 and 100 kW.18. The apparatus of claim 15, wherein the pump source is configured togenerate pump radiation having a peak laser intensity of greater than10⁵ W/cm2.
 19. A system comprising: a broadband source comprising: achamber configured to contain a volume of inert gas; a target materialsource positioned on a first side of the chamber; a debris collectorpositioned on a second side of the chamber opposite the target materialsource, wherein the target material source is configured to deliver astream of target material through a plasma-forming region of thechamber, wherein the debris collector is configured to collect targetmaterial, a pump source configured to deliver pump radiation to theplasma-forming region of the chamber, wherein the pump radiation issufficient to generate broadband radiation via formation of a plasma byexcitation of the target material within the plasma-forming region ofthe chamber; one or more focusing optical elements configured to focusthe pump radiation into the plasma-forming region; and one or morereflective collection optical elements configured to collect a portionof the broadband radiation from the plasma and deliver the portion ofthe broadband radiation to one or more optical elements external to thechamber through an aperture in a wall of the chamber; a set ofilluminator optics configured to direct the broadband radiation from theone or more reflective collection optics to one or more specimens; adetector; and a set of projection optics configured to receiveillumination from the surface of the one or more specimens and directthe illumination from the one or more specimens to the detector.
 20. Theapparatus of claim 19, wherein the broadband radiation comprises: atleast one of vacuum ultraviolet (VUV) or deep ultraviolet (DUV)radiation.
 21. The apparatus of claim 19, wherein the pump source, theone or more focusing optical elements and the reflective collectionoptical elements are arranged such that the broadband radiation has anumerical aperture higher than the NA of the pump radiation.
 22. Theapparatus of claim 19, wherein the pump source, the one or more focusingoptical elements and the reflective collection optical elements arearranged such that the broadband radiation has a numerical aperturelower than the NA of the pump radiation.
 23. The apparatus of claim 19,wherein the aperture in the wall of the chamber is windowless.
 24. Amethod comprising: delivering a stream of target material through aplasma-forming region of a gas chamber; collecting debris from theplasma-forming region; generating pump radiation; focusing the pumpradiation into the plasma-forming region of the chamber to generatebroadband radiation via formation of a plasma by excitation of thetarget material within the plasma-forming region of the chamber; andcollecting a portion of the broadband radiation from the plasma anddelivering the portion of the broadband radiation to one or more opticalelements external to the chamber through a windowless aperture in a wallof the chamber.